1. Field of the Invention
The present invention relates to an electrochromic display device and a display apparatus using the electrochromic display device.
2. Description of the Related Art
Recently, much research effort has been made to electronic paper as an electronic medium that is expected to displace paper. In contrast to conventional displays including CRTs and liquid crystal displays (LCDs), electronic paper requires the following characteristics: being a reflective display device; high white reflectivity and contrast ratio; high definition display; memory effect; low-voltage drive capability; slimness; lightness; and inexpensiveness, for example. In terms of its display characteristics, in particular, it is required that electronic paper have as excellent a white reflectivity and contrast ratio as paper does, and the development of a display device offering all of these characteristics is far from easy. Although electronic paper technologies have heretofore been proposed, including reflective liquid crystal devices, electrophoretic display devices and toner electrophoretic displays, they are all poor in white reflectivity.
A phenomenon in which electrochromic materials applied with a voltage show a reversible color change during the electrochemical redox reaction is called electrochromism. Electrochromic (hereinafter abbreviated to “EC” in some case display devices, which utilize color change in EC compounds that cause such a phenomenon, have emerged as a candidate for electronic paper because they serve as a reflective display device, have high white reflectivity as well as a memory effort and can be driven at low voltage. For example, Japanese Patent Application Laid-Open (JP-A) No. 2001-510590, Japanese Patent Application Laid-Open (JP-A) No. 2002-328401, and Japanese Patent Application Laid-Open (JP-A) No. 2004-151265 disclose EC devices in which an organic EC compound is attached to the surface of semiconductor fines particles made of, for example, titanium oxide. Such EC devices are known for their ability of efficiently producing or removing colors by utilizing a surface-area effect of the semiconductor fine particles, as well as for their excellent repetition durability. Their memory effort however, lasts as short as 20 minutes or so while a voltage is not applied, thereby posing problems related to the improvement of their image-retaining properties.